function stepFun = continuousStepFunction(t, rampRate)
    %  The Continuous Step Function is based on the following function
    % 
    %                              1
    %          stepFun(t) = ---------------
    %                        exp(-R*t) + 1
    %
    %  The center of rising edge is stepFun(0)=0.5.
    %
    %
    % [Inputs]
    %   rampRate: Slope at the center of rising edge, in unit of [1/s]. (Default: 1e6 )
    %
    if nargin < 2
        rampRate = 1e6;
    end


%     p = inputParser;
%     p.addRequired('rampRate', @(x)isnumeric(x) && isscalar(x) && (x>0) ); 
%     p.parse(rampRate);

    R = 4*rampRate;  % based on the slope @ stepFun = 0.5
    stepFun = 1./(  exp( -R*t )  +  1  );
end